1. Field of the Invention
This invention relates to a scanning antenna diversity system for vehicles having a multi-antenna installation.
2. The Prior Art
Circuit arrangements for antenna diversity systems are known from U.S. Pat. No. 4,752,968. With an antenna diversity receiving system for eliminating interference in frequency-modulated (FM) radio transmissions, a diversity processor is provided with a number of antenna signals, wherein a selected antenna signal is switched through to the receiver at each interval in time. From this received signal, a signal is derived in the intermediate frequency (IF) range of the receiver by conversion and coupled to the diversity processor for interference detection. When an interference is detected, switching signals are derived in the diversity processor for reversing to another antenna signal. Therefore, audio frequency interference caused by multichannel reception is largely avoided if adequate reception conditions are present.
However, in areas having poor reception conditions, U.S. Pat. No. 4,752,968 discloses circuit arrangements which have frequent reversing or switchover processes. These circuit arrangements have interference caused by such changeovers, particularly in areas where the signals are weak, and the reception quality in the motor vehicle is good. The audio quality is thus often impaired by these switchover interferences.
With frequency-modulated (FM) radio signals (for example ultra shortwave reception or television audio reception), reception interference can occur in connection with an antenna actually locked on to, the interference being caused by superimposing a plurality of partial waves with different amplitude, phase, and transmission time differences at their reception location. The level breakdowns caused by this superpositioning are correlated with frequency interference peaks, and cause signal distortions depending upon the modulation level in the audio frequency range. The minimum test time therefore corresponds with the interference detection time. If the interference detector detects interference or noise, the diversity processor induces further switching, and the further antenna signals and their linear combinations formed in an antenna matrix, if any, are all tested. If all available HF-signals show interferences, all HF-signals are continually and successively switched through to the receiver. Since this search process is repeated in rapid sequence, an audible interference signal in the form of a crackling noise is superimposed on the received signal on the low-frequency level, under special received conditions.
Therefore, measures are implemented in DE 440 03 612 in order to limit the frequency of further or continuing switching, in particularly, unfavorable receiver positions. This is accomplished, according to this patent, with the help of a stop device which prevents further switching when there is frequent interference indicated by the diversity processor, in a way that is adapted to the conditions of the reception. The interferences caused by reversing are in fact reduced by this measure. However, the reception interference indicated by the diversity processor naturally has an interference effect in the received signal.
Therefore, an object of the present invention is to provide for a scanning antenna diversity system, an apparatus which reduces further switching, and consequently the interferences and noise connected with this further switching.
In the invention, there is provided a scanning diversity antenna system for motor vehicles having a multi-antenna installation, with at least two antennas. The received voltages from the antenna signals are formed as linear combinations, and supplied to a controllable logic switching element. The received signal, which varies in terms of diversity, is switched through in different switching positions to a receiver. The receiver controls a diversity processor with a received signal derived from the varying received signals, the diversity processor cyclically switching the logic element to another switching position during reception interference. The received antenna signals are additionally supplied, to a maximum-level indicator, isolated from the signal path of the received signal, where the peak levels of the antenna signals, and thus the associated maximum-level switching position of the logic switching element are continuously determined. These determinations are carried out by a comparison of the received signal that is switched to the actual receiver, without any auxiliary modulation or the addition of any auxiliary signal. The peak level switching position is continually updated as an address signal at the output of the maximum-level indicator, and also supplied to the logic switching element. The logic switching element is designed so that in the cyclic through-switching of the received signals to the receiver, and while testing for lack of interference, a reversing position is available, in addition to the discrete switching positions of the logic switching element, for switching over to the maximum-level switching mode. The reversing position is used at least once during each cyclic pass through the switching positions. The cyclic stepping mode is interrupted, and the logic switching element continually switches the strongest signal received through to the receiver, with the help of the address signal, for the peak switching position. In the event that an interference is indicated in the diversity processor, the system reverses from the maximum-level switching mode back to the stepping mode.
The invention has the advantage that the received signal selected by the diversity processor is available at any time in the reverse position, and provides the strongest signal level due to pre-selection of the received signals so that there is low probability of interference. Therefore, the advantage is that the pre-selected received signal has an interference probability less than with the discrete signals, and thus there is a lower rate of interference detection of the diversity processor. On the other hand, in FM radio transmissions, extremely strong. interferences may occur, even with large received signals, such as during multichannel reception with large differences in transmission time, or due to interferences with an adjacent or even the same channel. Therefore, it is necessary that the selection, based on minimal interference, retains priority,and that the diversity processor also tests the maximum-level signals for freedom from interference, and, if need be, switches to a signal present in the discrete form.
Another advantage of the invention is that it is no longer necessary to invade the signal chain of the receiver connected to the loudspeakers to find the received signal with the highest level, since the available received signals are supplied to the maximum level indicator by way of the level comparator switch.
Other objects and features of the present invention will become apparent from the following detailed description considered in connection with the accompanying drawings which disclose several embodiments of the inventions. It is to be understood, however, that the drawings are designed for the purposes of illustration only, and not as a definition of the limits of the invention.